I. Field of the Invention
The present invention relates to an image-inputting element, so called pressure-sensitive tablet. The pressure-sensitive tablet is used as a part of an image-inputting device of communication equipment.
II. Description of the Related Art
The pressure-sensitive tablets are elements used for inputting images at positions touched by a finger or a writing tool, such as a ball-point pen or pencil. Conventional pressure-sensitive tablets have a laminated structure comprising an electro-conductive substrate, an electro-conductive film superposed on the substrate, and a spacer interposed therebetween. The spacer is usually in the form of particles or granules, and electrodes are disposed on the film and the substrate. The electro-conductive film and the electro-conductive substrate are normally insulated by the spacer. Upon pressing the electro-conductive film with a finger or a writing tool, the pressed portion of the film is deformed to contact the electro-conductive substrate. Thus, only the pressed portion is made electro-conductive. The position of the pressed portion, i.e., the portion which became electro-conductive is calculated from the relationship between the resistance and the distance from the electro-conductive portion to the electrodes. Since the pressure-sensitive tablets are strong against outer noise, simple and can be compact, they are now widely used in, for example, computerized image-inputting processors, automatic teller machines and the like.
Typical conventional pressure-sensitive tablets are the so called dot-matrix type. That is, the spacer of the tablets is in the form of particles of cylindrical shape having a diameter of, e.g., 150 .mu.m and a length of, e.g., 200 .mu.m. At least one of the upper and lower surfaces of the cylinder is fixed to the electro-conductive film or the electro-conductive substrate, respectively. Since the particles are relatively large, they can be seen and they can be felt when the film is touched by a finger. Thus, the outer appearance of the tablets is bad and they produce a bad feeling on the finger when touching the tablet.
To overcome these drawbacks, Japanese Patent Disclosure (Kokai) No. 188726/84 discloses a tablet in which the spacer is in the form of a rubber sheet containing short and, fine metal filaments embedded therein in parallel with the thickness direction of the rubber sheet. When the film is pressed, the edges of the filament are made to contact the electro-conductive film and the substrate to make the pressed portion electro-conductive. However, this tablet has a drawback in that the durability is low because the edges of the metal filaments damage the electro-conductive film and the substrate.
U.S. Pat. No. 3,911,215 discloses a tablet in which the spacer is in the form of particles having truncated spherical shape. The particles are made of a material such as undeformable epoxy. However, since the particles do not deform, the film and the substrate are damaged by the particles, so that the durability, of the tablet is low. Further, since the particles do not deform, if a point of the film very close to a particle is pressed with a sharp object such as a ball-point pen, the film will not contact the electro-conductive substrate because the particle prevents such contact. Therefore, for example, if a circle line is written, the line is jagged by the existence of the particles, and thus the sensitivity is low.
Further, the resistance of the electro-conductive film of the conventional tablets gradually changes with time, especially when they are used under the sunlight or under a strong fluorescent lamp. As a result, the pressed portion calculated by the computer, shifts from the actual portion pressed.